Memcache
It's also good to note that add will succeed if the key exists but is expired
Memcache::add
(PECL memcache >= 0.2.0)
Memcache::add — Añadir un valor en el servidor
Descripción
Memcache::add() guarda la variable
var con la clave key solamente si
la clave no existe todavía en el servidor.
También se puede usar la función memcache_add().
Parámetros
-
key -
La clave que quedará asociada con el valor.
-
var -
La variable a guardar. Cadenas y enteros se guardan tal y como son, otros tipos se guardan serializados.
-
flag -
Use
MEMCACHE_COMPRESSEDpara guardar datos comprimidos (usa zlib). -
expire -
Tiempo de expiración del valor. Si es iqual a cero, el valor nunca expirará. También puedes usar Unix timestamp o el número de segundos empezando desde la fecha actual, el valor no puede exceder de 2592000 segundos (30 días).
Valores devueltos
Devuelve TRUE en caso de éxito o FALSE en caso de error.
Devuelve FALSE si la clave ya existe. Por el resto
Memcache::add() se comporta similar a
Memcache::set().
Ejemplos
Ejemplo #1 Ejemplo Memcache::add()
<?php
$memcache_obj = memcache_connect("localhost", 11211);
/* procedural API */
memcache_add($memcache_obj, 'var_key', 'test variable', false, 30);
/* OO API */
$memcache_obj->add('var_key', 'test variable', false, 30);
?>
Ver también
- Memcache::set() - Guarda datos en el servidor
- Memcache::replace() - Reemplaza el valor de un valor existente
add a note
User Contributed Notes
Memcache::add - [7 notes]
ktamas77 at gmail dot com ¶
4 years ago
skeleton of a thread safe updater for an incremental counter:
<?php
$key = "counter";
$value = $memcache->increment($key, 1);
if ($value === false) {
// --- read from DB ---
$query = "SELECT value FROM database";
$result = mysql_query($query);
$row = mysql_fetch_assoc($result);
$db_value = $row["value"];
$add_value = $memcache->add($key, $db_value + 1, 0, 0);
if ($add_value === false) {
$value = $memcache->increment($key, 1)
if ($value === false) {
error_log ("counter update failed.");
}
} else {
$value = $db_value + 1;
}
}
// --- display counter value ---
echo $value;
?>
roberto at spadim,com dot br ¶
5 years ago
[c.2007]
if you read source code for MMC_SERIALIZED you will see at line ~1555 that [a line ~1560]
!(is_string,is_long,is_double,is_bool)
[is] serialized and that serialized values are flaged as MMC_SERIALIZED for return (fetch) code unserialize these values again
Davide Renzi ¶
3 years ago
Race conditions happen on an heavy load server when more than one thread tries to execute memcache_add.
For example if thread A and thread B try to save the same key you can test that sometimes both return TRUE.
To have the right behaviour you can verify that the correct value is in the assigned key:
<?php
function memcache_safeadd(&$memcache_obj, $key, $value, $flag, $expire)
{
if (memcache_add($memcache_obj, $key, $value, $flag, $expire))
{
return ($value == memcache_get($memcache_obj, $key));
}
return FALSE;
}
?>
duerra at nospam dot yahoo dot com ¶
2 years ago
If you're interested in using compression, please note that, at least for PHP version 5.3.2 and Memcache version 3.0.4, when retrieving a key who's value is a numeric or boolean type, PHP throws a notice of the following:
Message: MemcachePool::get(): Failed to uncompress data
The way around this is to test your variable type before setting or adding it to Memcache, or even cast it as a string.
<?php
$key = 'mc_key';
$value = 12345;
$compress = is_bool($value) || is_int($value) || is_float($value) ? false : MEMCACHE_COMPRESSED;
$mc= new Memcache;
$mc->connect('localhost', 11211);
$mc->add($key, $value, $compress);
echo $mc->get($key);
//Alternative is to cast the variable
$value = is_scalar($value) ? (string)$value : $value;
$mc->add($key, $value, MEMCACHE_COMPRESSED);
?>
rune(at)intermedia(dot)no ¶
4 years ago
Key may not exceed 250 chars according to memcached protocol.
php at tapirpirates dot net ¶
5 years ago
memcache has no locking mechanism, but you could implement it manually.
basic locking through the add method:
<?php
// locks time out after 5 seconds
Define( 'LOCK_TIMEOUT', 5 );
$lock = $memcache->add( 'lock:' . $key, 1, false, LOCK_TIMEOUT );
if ( $lock ) {
// no lock on this key, so do what you want
$value = $memcache->get( $key );
$memcache->set( $key, $value+1 );
// release lock
$memcache->delete( 'lock:' . $key );
}
else {
// variable is currently locked, so do something else
}
?>
furthermore, you could implement a loop which checks if there is a lock, and if there is, wait some time and try again, until the lock is free.
remember: locking will heavily increase your memcache hits and obviously is not what memcache is made for. altough it's not possible for a lock to be forgotten (there's a timeout after all) there is the possibility to get locked out for a very long time.
an alternative may be to implement locking through apc_add (or shared memory), but i've never tried it.
if you absolutley have to implement locks, memcached is probably the wrong solution anyway.